DPF Regeneration
A known system for treating exhaust gas passing through an exhaust system of a diesel engine comprises a diesel oxidation catalyst (DOC) that oxidizes hydrocarbons (HC) to CO2 and H2O and converts NO to NO2, and a diesel particulate filter (DPF) that traps diesel particulate matter (DPM). DPM includes soot or carbon, the soluble organic fraction (SOF), and ash (i.e. lube oil additives etc.). The DPF is located downstream of the DOC in the exhaust gas flow. The combination of these two exhaust gas treatment devices prevents significant amounts of pollutants such as hydrocarbons, carbon monoxide, soot, SOF, and ash, from entering the atmosphere. The trapping of DPM by the DPF prevents black smoke from being emitted from a vehicle's exhaust pipe.
The DOC oxidizes hydrocarbons (HC) and converts NO to NO2. The organic constituents of trapped DPM within the DPF, i.e., carbon and SOF, are oxidized within the DPF, using the NO2 generated by the DOC, to form CO2 and H2O, which can then exit the exhaust pipe to atmosphere.
The rate at which trapped carbon is oxidized to CO2 is controlled not only by the concentration of NO2 or O2 but also by temperature. Specifically, there are three important temperature parameters for a DPF.
The first temperature parameter is the oxidation catalyst's “light off” temperature, below which catalyst activity is too low to oxidize HC. Light off temperature is typically around 250° C.
The second temperature parameter controls the conversion of NO to NO2. This NO conversion temperature spans a range of temperatures having both a lower bound and an upper bound, which are defined as the minimum temperature and the maximum temperature at which 40% or greater NO conversion is achieved. The conversion temperature window defined by those two bounds extends from approximately 250° C. to approximately 450° C.
The third temperature parameter is related to the rate at which carbon is oxidized in the filter. Reference sources in relevant literature call that temperature the “Balance Point Temperature” (or BPT). It is the temperature at which the rate of oxidation of particulate, also sometimes referred to as the rate of DPF regeneration, is equal to the rate of accumulation of particulate. The BPT is one of the parameters that determine the ability of a DPF to enable a diesel engine to meet expected tailpipe emissions laws and/or regulations.
Typically, a diesel engine runs relatively lean and relatively cool compared to a gasoline engine. That factor makes natural achievement of BPT problematic.
Therefore, a DPF requires regeneration from time to time in order to maintain particulate trapping efficiency. Regeneration involves the presence of conditions that will burn off trapped particulates whose unchecked accumulation would otherwise impair DPF effectiveness. While “regeneration” refers to the general process of burning off DPM, two particular types of regeneration are recognized by those familiar with the regeneration technology as presently being applied to motor vehicle engines.
The term “passive regeneration” is generally understood to mean regeneration that can occur anytime that the engine is operating under conditions that burn off DPM without initiating a specific regeneration strategy embodied by algorithms in an engine control system. The term “active regeneration” is generally understood to mean regeneration that is initiated intentionally, either by the engine control system on its own initiative or by the driver causing the engine control system to initiate a programmed regeneration strategy, with the goal of elevating temperature of exhaust gases entering the DPF to a range suitable for initiating and maintaining burning of trapped particulates.
The creation of conditions for initiating and continuing active regeneration, whether forced or not, generally involves elevating the temperature of exhaust gas entering the DPF to a suitably high temperature.
Intended Exothermic Reaction-DPF Regeneration
There are several methods for initiating a forced regeneration of a DPF such as retarding the start of main fuel injections or post-injection of diesel fuel to initiate an exothermic reaction in the DOC to elevate exhaust gas temperatures entering the DPF while still leaving excess oxygen for burning the trapped particulate matter.
These methods are able to increase the exhaust gas temperature sufficiently to elevate the catalyst's temperature above catalyst “light off” temperature and provide excess HC that can be oxidized by the catalyst. Such HC oxidation is an intended exothermic reaction that provides the necessary heat to raise the temperature in the DPF above the BPT.
The amount of HC and CO generated by the engine may sometime exceed the stoichiometric quantity of NOx that is to be reduced over the catalyst. This leads to HC and CO breakthroughs at the DOC outlet (“HC/CO slip”), wherein the HC/CO slip cannot be oxidized to CO2 and H2O.
Unintended Exothermic Reaction
The present inventors have recognized that an unintended exothermal condition may present an elevated risk of permanent damage to after-treatment components.
The present inventors have recognized an unintended exothermal condition can be caused by different conditions including engine misfiring, incorrect calibration of engine controls, damaged injector, excessive soot accumulation in the DPF and passive regeneration, HC slipping, or malfunctioning external fuel dosing systems.